Demons are spirit creatures who sinned against God according to 2 Peter 2:4 these one rebelled against God, since they are spirit creatures this explains why you can’t see them. The first angel made himself into a demon Satan the Devil, the Bible also refers to him as a “ruler of the demons” according to Matthew 12:24, 26.

If you saw a demon you would turn instantly religious because of the sheer frightening spectacle before you and that's the last thing generally that a demon wants to do is turn people religious. An enemy is more powerful if they remain in stealth mode. Tactical warfare